paralysis of the extraocular muscles that control the movements of the eye. Ophthalmoplegia usually involves the third (oculomotor), fourth (trochlear), or sixth (abducens) cranial nerves. Double vision is the characteristic symptom in all three cases. In oculomotor paralysis the muscles controlling the eye are affected in such a way that the eye drifts outward and slightly downward and has difficulty turning inward and upward. In addition, the upper eyelid of the affected eye usually droops, a condition called ptosis, and the pupil may be enlarged. If the pupil is abnormally large, the possibility of a cerebral aneurysm arises. This can be associated with pain. Trochlear paralysis, involving another muscle, the superior oblique, causes a vertical deviation of the affected eye. Abducens nerve paralysis affects still another ocular muscle, the lateral rectus, such that the affected eye turns inward toward the nose and cannot fully turn outward.